Traditionally, topically administered medicine was often formulated as liquids. Applying a liquid to a skin surface often resulted in a portion of the dose spreading beyond the target area. Cream-based formulations were developed as viscous liquids to prevent the unintended application of the medicine to an unaffected area. More recently, pharmacists have been taking traditional medicines and “compounding” them in a cream base.
Administering the cream-based medicines is a challenge because providing an accurate measured dose is not easy. One common form of a dispenser is a traditional hypodermic syringe, without the needle. The user can depress the plunger to force an amount of cream out of the barrel as indicated by markings on the side of the barrel. For older patients, it is not always easy to measure out 0.1 ml or so of medicine, as this may require more dexterity than is available.
Recent improvements have been made to develop a more accurate method of metering creams, such as is described in U.S. Pat. No. 7,213,994. Specifically, the improvements comprise a metering system that uses a plunger to express an accurate dosage from a chamber by relying on a specified number of turns of a drive screw to advance the plunger and deliver a known amount of medicine. The metering system provides an audible sound when a unit or “click” has been delivered.
To ensure that the dispenser provides an accurate dosage, the patient should be consistently alerted to stop rotation of the drive screw at the appropriate location, and the amount of medicine that is pushed through a dispensing end should not vary due to leaks or fluctuation in the movement of the plunger.
In certain cases, particularly when there is not a perfectly tight fit between the plunger and the walls of the chamber, the plunger may turn slightly within the chamber when the drive screw turns, which may allow small gaps to form between the plunger and the chamber. As a result, the cream may squeeze down through the small gaps rather than exiting through the dispensing end of the chamber, which could result in metering inaccuracies. Because of the potential for possible leaks with cream-based medicines, this design may not be practical for use with certain liquids or fluids that would have a higher tendency to leak or spill.
In some embodiments, the coupling between the drive screw and the chamber may have some elasticity when the drive screw is retained within the chamber through the use of internally directed tapering fingers, which allows the chamber to be pushed up slightly relative to the drive screw when the base is rotated. This slight shift may cause less than the full dosage to be dispensed, which could also lead to metering inaccuracies.
Also, in many cases, the patient is relying on an audible sound to confirm that the dosage is complete. In certain instances, the click mechanism may lose elasticity if left in a “mid-click” position. In those cases, it may take some time for the click mechanism to regain its elasticity and until it does, there may not be an audible click response, which could also potentially lead to metering inaccuracies.
As a result, it may be desirable to provide a metered dispenser that forms a fluid seal for a variety of compositions, including viscous liquids or other fluids with a positive viscosity, which also includes a click mechanism that is not affected when the dispenser is left in a “mid-click” position.